Probably a buggy driver or service.
Could be for anything: modem, printer, net card, sound card, graphics card. It could be a device which doesn't work properly, e.g. a CD-RW which is configured to use DMA although it doesn't fully support it (e.g. TDK 12/10/32).
Uninstall all non-critical drivers, and disable all services - then use standby, each time restarting one service or adding one driver - repeat until you find the culprit, then either update the driver, or replace the hardware.
This was happening on my system due to a Lexmark printer - I was using the latest driver, so in the end I had to disable standby because I didn't want to get rid of the printer.